The cholera bacterium was first identified as a disease-causing agent through the groundbreaking work of Dr. John Snow in London in 1854. Dr. Snow mapped the incidence of disease and found that it clustered around a particular water pump. How did he make the causal connection between that pump and the disease

Dr. John Snow in London in 1854 discovered that the water where the cholera outbreak was prevalent was the same water used in other places. He came to a conclusion that it must be as a result of the pipes transporting the water. The pipes were found to have sewage surrounding them which made the water contaminated.

When the pipes were removed from the sewage surroundings the contamination, sickness and death rates reduced drastically.

you know they are heterozygous for a trait because they have it as jj or jj.